Method and apparatus of receiving data in wireless communication system

ABSTRACT

A method and an apparatus of receiving data in a wireless communication system. The method includes receiving a downlink (DL) grant on a physical downlink control channel (PDCCH) through a first DL component carrier (CC), and receiving data based on the DL grant through a second DL CC.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and an apparatus of receiving data in awireless communication system.

BACKGROUND ART

In wireless communication systems, one base station (BS) generallyprovides services to a plurality of user equipments (UEs). The BSschedules user data for the plurality of UEs, and transmits the userdata together with control information containing scheduling informationfor the user data. In general, a channel for carrying the controlinformation is referred to as a control channel, and a channel forcarrying the user data is referred to as a data channel. The UE findscontrol information of the UE by searching for the control channel, andprocesses data of the UE by using the control information.

In order for the UE to receive user data assigned to the UE, controlinformation for the user data on a control channel must be received. Ina given bandwidth, a plurality of pieces of control information for aplurality of UEs are generally multiplexed within one transmissioninterval. That is, to provide a service to the plurality of UEs, the BSmultiplexes the plurality of pieces of control information for theplurality of UEs and then transmits the control information through aplurality of control channels. The UE searches for control channel ofthe UE among the plurality of control channels.

Blind decoding is one of schemes for detecting specific controlinformation from the plurality of pieces of multiplexed controlinformation. The blind decoding attempts to recover a control channel byusing several combinations of information in a state where a UE has noinformation required to recover the control channel. That is, in a statewhere the UE does not know whether control information transmitted fromthe BS is control information of the UE and the UE does not know inwhich portion the control information of the UE exists, the UE decodesall pieces of given control information until the control information ofthe UE is found. The UE can use information unique to each UE to detectthe control information of the UE. For example, when the BS multiplexescontrol information of each UE, an identifier unique to each UE can betransmitted by being masked onto a cyclic redundancy check (CRC). TheCRC is a code used for error detection. The UE de-masks uniqueidentifier of the UE from the CRC of the received control information,and then can detect the control information of the UE by performing CRCchecking.

Meanwhile, as a mobile communication system of a next generation (i.e.,post-3rd generation), an international mobile telecommunication-advanced(IMT-A) system is standardized aiming at support of an Internet protocol(IP)-based seamless multimedia service in an internationaltelecommunication union (ITU) by providing a high-speed transmissionrate of 1 gigabits per second (Gbps) in downlink communication and 500megabits per second (Mbps) in uplink communication. In a 3rd generationpartnership project (3GPP), a 3GPP long term evolution-advanced (LTE-A)system is considered as a candidate technique for the IMT-A system. TheLTE-A system is evolved to increase a completion level of the LTEsystem, and is expected to maintain backward compatibility with the LTEsystem. This is because the provisioning of compatibility between theLTE-A system and the LTE system is advantageous in terms of userconvenience, and is also advantageous for a service provider sinceexisting equipment can be reused.

In general, a wireless communication system is a single carrier systemsupporting a single carrier. The transmission rate is proportional totransmission bandwidth. Therefore, for supporting a high-speedtransmission rate, transmission bandwidth shall be increased. However,except for some areas of the world, it is difficult to allocatefrequencies of wide bandwidths. For effectively using fragmented smallfrequency bands, a spectrum aggregation (also referred to as bandwidthaggregation or carrier aggregation) technique is being developed. Thespectrum aggregation technique is to obtain the same effect as if whicha frequency band of a logically wide bandwidth may be used byaggregating a plurality of physically discontiguous frequency bands in afrequency domain. Through the spectrum aggregation technique, multiplecarrier (multi-carrier) can be supported in the wireless communicationsystem. The wireless communication system supporting multi-carrier isreferred to as a multi-carrier system. The carrier may be also referredto as a radio frequency (RF), component carrier (CC), etc.

Accordingly, there is a need for a method and an apparatus ofeffectively receiving data in a multi-carrier system.

DISCLOSURE OF INVENTION Technical Problem

The present invention provides a method and an apparatus of receivingdata in a wireless communication system.

Solution to Problem

In an aspect, a method of receiving data in a wireless communicationsystem, carried in a user equipment (UE), is provided. The methodincludes receiving a downlink (DL) grant on a physical downlink controlchannel (PDCCH) through a first DL component carrier (CC) from a basestation (BS), and receiving data based on the DL grant through a secondDL CC from the BS.

Preferably, the DL grant comprises a CC indication field indicating thesecond DL CC.

Preferably, a cyclic redundancy check (CRC) of the DL grant is scrambledwith a UE identifier (ID), and the UE ID indicates the second DL CC.

Preferably, an index of a control channel element (CCE) indicates thesecond DL CC, the CCE is used for transmitting the PDCCH.

The method may further includes receiving a second DL grant on a secondPDCCH through the first DL CC from the BS, and receiving second databased on the second DL grant through a third DL CC.

In another aspect, a method of transmitting data in a wirelesscommunication system, carried in a UE, is provided. The method includesreceiving a uplink (UL) grant on a PDCCH through a first DL CC from aBS, and transmitting data based on the UL grant through a first UL CC tothe BS.

Preferably, the UL grant comprises a CC indication field indicating thefirst UL CC.

In still another aspect, a UE is provided. The UE includes a radiofrequency (RF) unit transmitting and/or receiving a radio signal and aprocessor coupled with the RF unit and configured to receive a DL granton a PDCCH through a first DL CC, and receive data based on the DL grantthrough a second DL CC.

Preferably, the DL grant comprises a CC indication field indicating thesecond DL CC.

Advantageous Effects of Invention

A method and an apparatus of effectively receiving data are provided.Accordingly, overall system performance can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a wireless communication system.

FIG. 2 shows an example of a plurality of component carriers (CCs) usedin a multi-carrier system.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

FIG. 4 shows an example of a plurality of physical channels (PHYs).

FIG. 5 shows an example of a bandwidth used by a PHY.

FIG. 6 shows an example of an asymmetric structure of downlink anduplink in a multi-carrier system.

FIG. 7 shows a structure of a radio frame.

FIG. 8 shows an example of a resource grid for one downlink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a frequencydivision duplex (FDD) system.

FIG. 10 shows an example of an resource element group (REG) structurewhen a base station (BS) uses one or two transmit (Tx) antennas.

FIG. 11 shows an example of an REG structure when a BS uses four Txantennas.

FIG. 12 shows an example of mapping of a physical control formatindicator channel (PCFICH) to REGs.

FIG. 13 is a flow diagram showing an example of a method of transmittingdata and receiving data performed by a user equipment (UE).

FIG. 14 is a flowchart showing an example of a method of configuring aphysical downlink control channel (PDCCH).

FIG. 15 shows an example of a method of multiplexing a plurality ofPDCCHs for a plurality of UEs, performed by a BS.

FIG. 16 shows an example of a method of monitoring a control channel,performed by a UE.

FIG. 17 shows an example of a PDCCH transmission method in amulti-carrier system.

FIG. 18 is a flow diagram showing method of receiving data, performed bya UE, according to an embodiment of the present invention.

FIG. 19 is a flow diagram showing method of transmitting data, performedby a UE, according to another embodiment of the present invention.

FIG. 20 illustrates a method of configuring a multi-PDCCH.

FIG. 21 shows an example in which a multi-PDCCH is transmitted in amulti-carrier system.

FIG. 22 shows another example in which a multi-PDCCH is transmitted in amulti-carrier system.

FIG. 23 shows still another example in which a multi-PDCCH istransmitted in a multi-carrier system.

FIG. 24 shows an example in which PDCCHs are transmitted in amulti-carrier system.

MODE FOR THE INVENTION

FIG. 1 is a block diagram showing a wireless communication system.

Referring to FIG. 1, a wireless communication system 10 includes atleast one base station (BS) 11. The BSs 11 provide communicationservices to specific geographical regions (generally referred to ascells) 15 a, 15 b, and 15 c. The cell can be divided into a plurality ofregions (referred to as sectors). A user equipment (UE) 12 may be fixedor mobile, and may be referred to as another terminology, such as amobile station (MS), a user terminal (UT), a subscriber station (SS), awireless device, a personal digital assistant (PDA), a wireless modem, ahandheld device, etc. The BS 11 is generally a fixed station thatcommunicates with the UE 12 and may be referred to as anotherterminology, such as an evolved node-B (eNB), a base transceiver system(BTS), an access point, etc.

Hereinafter, a downlink (DL) denotes communication from the BS to theUE, and an uplink (UL) denotes communication from the UE to the BS. Inthe DL, a transmitter may be a part of the BS, and a receiver may be apart of the UE. In the UL, the transmitter may be a part of the UE, andthe receiver may be a part of the BS.

The wireless communication system supports multi-antenna. Thetransmitter may use a plurality of transmit (Tx) antennas, and thereceiver may use a plurality of receive (Rx) antennas. The Tx antenna isa logical or physical antenna used to transmit one signal or one stream,and the Rx antenna is a logical or physical antenna used to receive onesignal or one stream.

If the transmitter and the receiver use multi-antenna, the wirelesscommunication system may be called as multiple input multiple output(MIMO) system.

FIG. 2 shows an example of a plurality of component carriers (CCs) usedin a multi-carrier system.

Referring to FIG. 2, a multi-carrier system may use N CCs (CC #1, CC #2,. . . , CC #N). Although it is described herein that adjacent CCs arephysically discontiguous in a frequency domain, this is for exemplarypurposes only. Adjacent CCs may be physically contiguous in a frequencydomain

Therefore, a frequency band of a logically wide bandwidth may be used inthe multi-carrier system by aggregating a plurality of physicallydiscontiguous and/or contiguous CCs in a frequency domain.

In downlink, a BS concurrently can transmit information to one UEthrough one or more CCs. In uplink, the UE can also transmit data to theBS through one or more CCs.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

Referring to FIG. 3, each of a transmitter 100 and a receiver 200 uses NCCs (CC #1, CC #2, . . . , CC #N) in a multi-carrier system. A CCincludes one or more physical channels (hereinafter, simply referred toas PHYs). A wireless channel is established between the transmitter 100and the receiver 200.

The transmitter 100 includes a plurality of PHYs 110-1, . . . , 110-M, amulti-carrier multiplexer 120, and a plurality of Tx antennas 190-1, . .. , 190-Nt. The receiver 200 includes a multi-carrier demultiplexer 210,a plurality of PHYs 220-1, . . . , 220-L, and a plurality of Rx antennas290-1, . . . , 290-Nr. The number M of PHYs of the transmitter 100 maybe identical to or different from the number L of PHYs of the receiver200. Although it is described herein that each of the transmitter 100and the receiver 200 includes a plurality of antennas, this is forexemplary purposes only. The transmitter 100 and/or the receiver 200includes a single antenna.

The transmitter 100 generates Tx signals from information based on the NCCs, and the Tx signals are transmitted on M PHYs 110-1, . . . , 110-M.The multi-carrier multiplexer 120 combines the Tx signals so that the Txsignals can be simultaneously transmitted on the M PHYs. The combined Txsignals are transmitted through the Nt Tx antennas 190-1, . . . ,190-Nt. The Tx radio signals are received through the Nr Rx antennas290-1, . . . , 290-Nr of the receiver 200 through the wireless channel.The Rx signals are de-multiplexed by the multi-carrier demultiplexer 210so that the Rx signals are separated into the L PHYs 220-1, . . . ,220-L. Each of the PHYs 220-1, . . . , 220-L recovers the information.

The multi-carrier system may include one or more carrier modules. Thecarrier module upconverts a baseband signal to a carrier frequency to bemodulated onto a radio signal, or downconverts a radio signal to recovera baseband signal. The carrier frequency is also referred to as a centerfrequency. The multi-carrier system may use a plurality of carriermodules for each carrier frequency, or use a carrier module which canchange a carrier frequency.

FIG. 4 shows an example of a plurality of PHYs. FIG. 4 shows an exampleof N CCs consisting of M PHYs (PHY #1, PHY #2, . . . , PHY #M).

Referring to FIG. 4, each of M PHYs has a specific bandwidth (BW). AnPHY #m has a center frequency f_(c,m) and a bandwidth ofN_(IFFT,m)×Δf_(m) (where m=1, . . . , M). Herein, N_(IFFT,m) denotes aninverse fast Fourier transform (IFFT) size of the PHY #m, and Δf_(m)denotes a subcarrier spacing of the PHY #m. The IFFT size and thesubcarrier spacing may be different or identical for each PHY. Centerfrequencies of the respective PHYs may be arranged with a regularinterval or an irregular interval.

According to a UE or a cell, each PHY may use a bandwidth narrower thana maximum bandwidth. For example, if it is assumed that each PHY has amaximum bandwidth of 20 mega Hertz (MHz), and M is 5, then a fullbandwidth of up to 100 MHz can be supported.

FIG. 5 shows an example of a bandwidth used by a PHY.

Referring to FIG. 5, if it is assumed that a maximum bandwidth of thePHY is 20 MHz, the PHY can use a bandwidth (e.g., 10 MHz, 5 MHz, 2.5MHz, or 1.25 MHz) narrower than the maximum bandwidth. Regardless of abandwidth size used by the PHY in downlink, a synchronization channel(SCH) may exist in each PHY. The SCH is a channel for cell search. Thecell search is a procedure by which a UE acquires time synchronizationand frequency synchronization with a cell and detects a cell identifier(ID) of the cell. If the SCH is located in all downlink PHYs, all UEscan be synchronized with the cell. In addition, if a plurality ofdownlink PHYs are allocated to the UE, cell search may be performed foreach PHY or may be performed only for a specific PHY.

As such, a UE or a BS can transmit and/or receive information based onone or more PHYs in the multi-carrier system. The number of PHYs used bythe UE may be different from or equal to the number of PHYs used by theBS. In general, the BS can use M PHYs, and the UE can use L PHYs (M≧L,where M and L are natural numbers). Herein, L may differ depending on atype of the UE.

The multi carrier system can have several types of uplink and downlinkconfigurations. In a frequency division duplex (FDD) system or a timedivision duplex (TDD) system, a structure of downlink and uplink may bean asymmetric structure in which an uplink bandwidth and a downlinkbandwidth are different from each other. Alternatively, the structure ofdownlink and uplink may be configured in which an uplink bandwidth and adownlink bandwidth are identical to each other. In this case, thestructure of downlink and uplink may be configured to a symmetricstructure in which the same number of PHYs exist in both uplink anddownlink transmissions or an asymmetric structure in which the number ofPHY differs between uplink and downlink transmissions.

FIG. 6 shows an example of an asymmetric structure of downlink anduplink in a multi-carrier system. A transmission time interval (TTI) isa scheduling unit for information transmission. In each of the FDDsystem and the TDD system, a structure of downlink and uplink is anasymmetric structure. If the structure of downlink and uplink is anasymmetric structure, a specific link may have a higher informationthroughput. Therefore, system can be optimized flexibly.

Hereinafter, for convenience of explanation, it is assumed that a CCincludes one PHY.

All transmission/reception methods used in a single carrier system usingcan also be applied to each CC of the transmitter and the receiver in amulti-carrier system. In addition, it is desirable for the multi-carriersystem to maintain backward compatibility with the single carrier systemwhich is legacy system of the multi-carrier system, This is because theprovisioning of compatibility between the multi-carrier system and thesingle carrier system is advantageous in terms of user convenience, andis also advantageous for a service provider since existing equipment canbe reused.

Now, a single carrier system will be described.

FIG. 7 shows a structure of a radio frame.

Referring to FIG. 7, the radio frame consists of 10 subframes. Onesubframe consists of two slots. Slots included in the radio frame arenumbered with slot numbers #0 to #19. A time required to transmit onesubframe is defined as a TTI. For example, one radio frame may have alength of 10 milliseconds (ms), one subframe may have a length of 1 ms,and one slot may have a length of 0.5 ms.

The structure of the radio frame is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe may change variously.

FIG. 8 shows an example of a resource grid for one downlink slot.

Referring to FIG. 8, the downlink slot includes a plurality oforthogonal frequency division multiplexing (OFDM) symbols in a timedomain and NDL resource blocks (RBs) in a frequency domain. The OFDMsymbol is for expressing one symbol period, and may be referred to as anorthogonal frequency division multiple access (OFDMA) symbol or a singlecarrier-frequency division multiple access (SC-FDMA) symbol according toa multiple access scheme. The number N^(DL) of resource blocks includedin the downlink slot depends on a downlink transmission bandwidthconfigured in a cell. One RB includes a plurality of subcarriers in thefrequency domain.

Each element on the resource grid is referred to as a resource element(RE). Although it is described herein that one RB includes 7×12 resourceelements consisting of 7 OFDM symbols in the time domain and 12subcarriers in the frequency domain for example, the number of OFDMsymbols and the number of subcarriers in the RB are not limited thereto.Thus, the number of OFDM symbols and the number of subcarriers maychange variously depending on a cyclic prefix (CP) length, a subcarrierspacing, etc. For example, when using a normal CP, the number of OFDMsymbols is 7, and when using an extended CP, the number of OFDM symbolsis 6.

The resource grid for one downlink slot of FIG. 8 can be applied to aresource grid for an uplink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a FDDsystem.

Referring to FIG. 9, the radio frame includes 10 subframes, and eachsubframe includes two consecutive slots. When using a normal CP, thesubframe includes 14 OFDM symbols. When using an extended CP, thesubframe includes 12 OFDM symbols. A SCH is transmitted in every radioframe. The SCH includes a primary (P)-SCH and a secondary (S)-SCH. TheP-SCH is transmitted through a last OFDM symbol of a 1st slot of asubframe 0 and a subframe 5 in a radio frame. When using the normal CP,the P-SCH is an OFDM symbol 6 in the subframe, and when using theextended CP, the P-SCH is an OFDM symbol 5 in the subframe. The S-SCH istransmitted through an OFDM symbol located immediately before an OFDMsymbol on which the P-SCH is transmitted.

A maximum of three OFDM symbols (i.e., OFDM symbols 0, 1, and 2) locatedin a front portion of a 1st slot in every subframe correspond to acontrol region. The remaining OFDM symbols correspond to a data region.A physical downlink shared channel (PDSCH) can be assigned to the dataregion. Downlink data is transmitted on PDSCH.

Cntrol channels such as a physical control format indicator channel(PCFICH), a physical HARQ (hybrid automatic repeat request) indicatorchannel (PHICH), a physical downlink control channel (PDCCH) etc., canbe assigned to the control region.

Resource element groups (REGs) are used for defining the mapping of acontrol channel to resource elements.

FIG. 10 shows an example of an REG structure when a BS uses one or twoTx antennas. FIG. 11 shows an example of an REG structure when a BS usesfour Tx antennas. In FIGS. 10 and 11, it is assumed that a maximum ofthree OFDM symbols (i.e., OFDM symbols 0, 1, and 2) located in a frontportion of a 1st slot in a subframe are control regions.

Referring to FIGS. 10 and 11, Rp indicates a resource element which isused to transmit a reference signal (hereinafter referred to as an ‘RS’)through antenna p (pε{0, 1, 2, 3}). The RS may be also referred to as apilot. One REG is composed of four adjacent resource elements in thefrequency domain other than resource elements which are used for RStransmission. In the OFDM symbol 0 in the subframe, two REGs existwithin one resource block in the frequency domain. It is to be notedthat the above REG structures are only illustrative and the number ofresource elements included in the REG may change in various ways.

The PHICH carries an HARQ acknowledgement (ACK)/not-acknowledgement(NACK) for uplink data.

The PCFICH carries information about the number of OFDM symbols used fortransmission of PDCCHs in a subframe. Although the control regionincludes three OFDM symbols herein, this is for exemplary purposes only.According to an amount of control information, the PDCCH is transmittedthrough the OFDM symbol 0, or the OFDM symbols 0 and 1, or the OFDMsymbols 0 to 2. The number of OFDM symbols used for PDCCH transmissionmay change in every subframe. The PCFICH is transmitted through a 1stOFDM symbol (i.e., the OFDM symbol 0) in every subframe. The PCFICH canbe transmitted through a single antenna or can be transmitted through amulti-antenna using a transmit diversity scheme. When a subframe isreceived, the UE evaluates control information transmitted through thePCFICH, and then receives control information transmitted through thePDCCH.

The control information transmitted through the PCFICH is referred to asa control format indicator (CFI). For example, the CFI may have a valueof 1, 2, or 3. The CFI value may represent the number of OFDM symbolsused for PDCCH transmission in a subframe. That is, if the CIF value is2, the number of OFDM symbols used for PDCCH transmission in a subframeis 2. This is for exemplary purposes only, and thus informationindicated by the CFI may be defined differently according to a downlinktransmission bandwidth. For example, if the downlink transmissionbandwidth is less than a specific threshold value, the CFI values of 1,2, and 3 may indicate that the number of OFDM symbols used for PDCCHtransmission in the subframe is 2, 3, and 4, respectively.

The following table shows an example of a CFI and a 32-bit CFI codewordwhich generates by performing channel coding to the CFI.

Table 1

TABLE 1 CFI codeword CFI <b₀, b₁, . . . , b₃₁> 1 <0, 1, 1, 0, 1, 1, 0,1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0,1> 2 <1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1,0, 1, 1, 0, 1, 1, 0, 1, 1, 0> 3 <1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1,1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1> 4 <0, 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, (Reserved) 0,0, 0, 0, 0, 0, 0, 0, 0>

The 32-bit CFI codeword can be mapped to a 16 modulated symbols using aquadrature phase shift keying (QPSK) scheme. In this case, 16 resourceelements (or subcarriers) are used in PCFICH transmission. That is, 4REGs are used in PCFICH transmission.

FIG. 12 shows an example of mapping of a PCFICH to REGs.

Referring to FIG. 12, the PCFICH is mapped to 4 REGs, and the respectiveREGs to which the PCFICH are mapped are spaced apart from one another.An REG to which the PCFICH is mapped may vary according to the number ofresource blocks in a frequency domain. In order to avoid inter-cellinterference of the PCFICH, the REGs to which the PCFICH is mapped maybe shifted in a frequency domain according to a cell ID.

Now, a PDCCH will be described.

A control region consists of a set of control channel elements (CCEs).The CCEs are indexed 0 to N(CCE)−1, where N(CCE) is the total number ofCCEs constituting the set of CCEs in a downlink subframe. The CCEcorresponds to a plurality of REGs. For example, one CCE may correspondto 9 REGs. A PDCCH is transmitted on an aggregation of one or severalconsecutive CCEs. A PDCCH format and the possible number of bits of thePDCCH are determined according to the number of CCEs constituting theCCE aggregation. Hereinafter, the number of CCEs constituting the CCEaggregation used for PDCCH transmission is referred to as a CCEaggregation level. In addition, the CCE aggregation level is a CCE unitfor searching for the PDCCH. A size of the CCE aggregation level isdefined by the number of contiguous CCEs. For example, the CCEaggregation level may be an element of {1, 2, 4, 8}.

The following table shows an example of the PDCCH format, the number ofREGs and the number of PDCCH bits.

Table 2

TABLE 2 Number of PDCCH format CCE aggregation level Number of REGsPDCCH bits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

Control information transmitted on the PDCCH is referred to as downlinkcontrol information (DCI). The DCI transports uplink schedulinginformation, downlink scheduling information, or an uplink power controlcommand, etc. The downlink scheduling information is also referred to asa downlink grant, and the uplink scheduling information is also referredto as an uplink grant.

FIG. 13 is a flow diagram showing an example of a method of transmittingdata and receiving data performed by a UE.

Referring to FIG. 13, a BS transmits an uplink grant to a UE at stepS11. The UE transmits uplink data to the BS based on the uplink grant atstep S12. The uplink grant may be transmitted on a PDCCH, and the uplinkdata may be transmitted on a physical uplink shared channel (PUSCH). Arelationship between a subframe in which a PDCCH is transmitted and asubframe in which a PUSCH is transmitted may be previously set betweenthe BS and the UE. For example, if the PDCCH is transmitted in an nthsubframe in a FDD system, the PUSCH may be transmitted in an (n+4)thsubframe.

The BS transmits an downlink grant to the UE at step S13. The UEreceives downlink data from the BS based on the downlink grant at stepS14. The downlink grant may be transmitted on a PDCCH, and the downlinkdata may be transmitted on a PDSCH. For example, the PDCCH and the PDSCHare transmitted in the same subframe.

As described above, a UE shall receive DCI on PDCCH to receive downlinkdata from a BS or transmit uplink data to a BS.

DCI may use a different DCI format in accordance with usage. Forexample, a DCI format for an uplink grant and a DCI format for adownlink grant is different each other. A size and usage of DCI maydiffer according to a DCI format.

The following table shows an example of the DCI format.

Table 3

TABLE 3 DCI format Objectives 0 Scheduling of PUSCH 1 Scheduling of onePDSCH codeword 1A Compact scheduling of one PDSCH codeword 1BClosed-loop single-rank transmission 1C Paging, RACH response anddynamic BCCH 1D MU-MIMO 2 Scheduling of closed-loop rank-adapted spatialmultiplexing mode 2A Scheduling of open-loop rank-adapted spatialmultiplexing mode 3 TPC commands for PUCCH and PUSCH with 2 bit poweradjustments 3A TPC commands for PUCCH and PUSCH with single bit poweradjustments

Referring to above table, a DCI format 0 is used for PUSCH scheduling.The DCI format 0 is used for an uplink grant.

A DCI format 1 is used for scheduling of one PDSCH codeword. A DCIformat 1A is used for compact scheduling of one PDSCH codeword. A DCIformat 1B is used for compact scheduling of one PDSCH codeword in aclosed-loop rank 1 transmission mode. A DCI format 1C is used forpaging, random access channel (RACH) response, and dynamic broadcastcontrol channel (BCCH). A DCI format 1D is used for PDSCH scheduling ina multi-user (MU)-MIMO mode. A DCI format 2 is used for PDSCH schedulingin a closed-loop rank-adapted spatial multiplexing mode. A DCI format 2Ais used for PDSCH scheduling in an open-loop rank-adapted spatialmultiplexing mode. Each of from DCI format 1 to DCI format 2A is usedfor a downlink grant. However, the DCI format may differ according to ausage of DCI or transmission mode of a BS.

DCI formats 3 and 3A are used for transmission of a transmission powercontrol (TPC) command for a physical uplink control channel (PUCCH) anda PUSCH. The DCI formats 3 and 3A is used for an uplink power controlcommand.

Each DCI format consists of a plurality of information fields. The typeof information fields constituting a DCI format, the size of each of theinformation fields, etc. may differ according to the DCI format. Forexample, a downlink grant (or an uplink grant) includes resourceallocation field indicating radio resource. The downlink grant (or theuplink grant) may include further a modulation and coding scheme (MCS)field indicating modulation scheme and channel coding sheme. Inaddition, the downlink grant (or the uplink grant) may include furthervarious information fields.

FIG. 14 is a flowchart showing an example of a method of configuring aPDCCH.

Referring to FIG. 14, in step S21, a BS generates information bit streamin accordance with a DCI format. In step S22, the BS attaches a cyclicredundancy check (CRC) for error detection to the information bitstream. The information bit stream may be used to calculate the CRC. TheCRC is parity bits, and the CRC may be attached in front of theinformation bit stream or at the back of the information bit stream.

The CRC is masked with an identifier (referred to as a radio networktemporary identifier (RNTI)) according to an owner or usage of the DCI.The masking may be scrambling of the CRC with the identifier. Themasking may be an modulo 2 operation or exclusive or (XOR) operationbetween the CRC and the identifier.

If the DCI is for a specific UE, a unique identifier (e.g., cell-RNTI(C-RNTI)) of the UE may be masked onto the CRC. The C-RNTI may be alsoreferred to as a UE ID. The CRC can be masked with different RNTI exceptC-RNTI, such as paging-RNTI (P-RNTI) for a paging message, systeminformation-RNTI (SI-RNTI) for system information, random access-RNTI(RA-RNTI) for indicating random access response which is response ofrandom access preamble transmitted by a UE, etc.

In step S23, the BS generates coded bit stream by performing channelcoding to the information bit stream attached the CRC. The channelcoding scheme is not limited. For example, convolution coding scheme canbe used. The number of PDCCH bits may differ in accordance with channelcoding rate.

In step of S24, the BS generates rate matched bit stream by performingrate matching to the coded bit stream. In step of S25, the BS generatesmodulated symbols by modulating the rate matched bit stream. In step ofS26, the BS maps the modulated symbols to resource elements.

As described above, a method of configuring one PDCCH is explained.However, a plurality of control channels may be transmitted in asubframe. That is, a plurality of PDCCHs for several UEs can betransmitted by being multiplexed in one subframe. Generation ofinformation bit stream, CRC attachment, channel coding, and ratematching, etc. are performed independently for each PDCCH. Theaforementioned process of configuring the PDCCH of FIG. 14 can beperformed independently for each PDCCH.

FIG. 15 shows an example of a method of multiplexing a plurality ofPDCCHs for a plurality of UEs, performed by a BS.

Referring to FIG. 15, a CCE set constituting a control region in asubframe is constructed of a plurality of CCEs indexed from 0 toN(CCE)−1. That is, the number of CCEs is N(CCE). A PDCCH for a UE #1 istransmitted on a CCE aggregation with a CCE index 0 at a CCE aggregationlevel of 1. A PDCCH for a UE #2 is transmitted on a CCE aggregation witha CCE index 1 at a CCE aggregation level of 1. A PDCCH for a UE #3 istransmitted on a CCE aggregation with CCE indices 2 and 3 at a CCEaggregation level of 2. A PDCCH for a UE #4 is transmitted on a CCEaggregation with CCE indices 4, 5, 6, and 7 at a CCE aggregation levelof 4. A PDCCH for a UE #5 is transmitted on a CCE aggregation with CCEindices 8 and 9 at a CCE aggregation level of 2.

The CCEs are mapped to resource elements (REs) according to a CCE-to-REmapping rule. In this case, a PDCCH of each UE is mapped to the REs bybeing interleaved in the control region in the subframe. Locations ofthe REs to be mapped may change according to the number of OFDM symbolsused for transmission of the PDCCHs in the subframe, the number of PHICHgroups, the number of Tx antennas, and the frequency shifts.

The BS does not provide the UE with information indicating where a PDCCHof the UE is located in the subframe. In general, in a state where theUE does not know a location of the PDCCH of the UE in the subframe, theUE finds the PDCCH of the UE by monitoring a set of PDCCH candidates inevery subframe. Monitoring implies that the UE attempts decoding each ofthe PDCCH candidates according to all the monitored DCI formats. This isreferred to as blind decoding or blind detection. If no CRC error isdetected if the UE performs CRC checking after de-masking C-RNTI from aPDCCH candidate, it is regarded that the PDCCH candidate is detected bythe UE as the PDCCH of the UE.

In addition, the UE does not know at which CCE aggregation level thePDCCH of the UE is transmitted. Therefore, the UE needs to attemptdecoding on the set of PDCCH candidates for each of all possible CCEaggregation levels.

FIG. 16 shows an example of a method of monitoring a control channel,performed by a UE.

Referring to FIG. 16, a CCE set constituting a control region in asubframe is constructed of a plurality of CCEs indexed from 0 toN(CCE)−1. That is, the number of CCEs is N(CCE). There are four types ofa CCE aggregation level L, that is, {1, 2, 4, 8}. A set of PDCCHcandidates monitored by the UE is differently defined according to theCCE aggregation level. For example, if the CCE aggregation level is 1,the PDCCH candidates correspond to all CCEs constituting the CCE set. Ifthe CCE aggregation level is 2, the PDCCH candidates correspond to a CCEaggregation with CCE indices 0 and 1, a CCE aggregation with CCE indices2 and 3, and so on. If the CCE aggregation level is 4, the PDCCHcandidates correspond to a CCE aggregation with CCE indices 0 to 3, aCCE aggregation with CCE indices 4 to 7, and so on. If the CCEaggregation level is 8, the PDCCH candidates correspond to a CCEaggregation with CCE indices 0 to 7, and so on.

A frame structure of a single carrier system, a PDCCH transmission andmonitoring method, etc., have been described above. For optimization ofa multi-carrier system, a multi-antenna scheme or a control channelshall be designed by considering a frequency channel property for eachCC. Therefore, it is important to properly use a system parameter and anoptimal transmission/reception scheme for each CC. In addition, the sameframe structure as a legacy system may be used in one CC of themulti-carrier system. In this case, the control channel shall beproperly modified to operate both a UE for the legacy system and a UEfor the multi-carrier system. Hereinafter, the UE for the legacy systemis referred to as a long term evolution (LTE) UE, and the UE for themulti-carrier system is referred to as an LTE-advanced (LTE-A) UE.

FIG. 17 shows an example of a PDCCH transmission method in amulti-carrier system.

Referring to FIG. 17, a UE uses two downlink component carriers (DL CCs)CC #1 and CC #2. In a type 1, the BS can transmit a PDCCH to the UEthrough several DL CCs. The PDCCH transmitted through the CC #1 maycarry the scheduling information of downlink data which is transmittedthrough the CC #1 or may carry the scheduling information of downlinkdata which is transmitted through the CC #2. That is, the schedulinginformation of downlink data which is transmitted through one of aplurality of DL CCs may be transmitted over the plurality of DL CCs.Accordingly, in the type 1, the PDCCH can obtain a frequency diversitygain. However, if the channel state of a specific DL CC is not good, thePDCCH transmitted through the specific DL CC may not be detected. Inthis case, the UE may not receive downlink data which is transmittedthrough a PDSCH corresponding to the PDCCH.

In a type 2, the BS can transmit a PDCCH to the UE through only any oneof several DL CCs. In the type 2, limited radio resources can beefficiently used because a control region through which the PDCCH istransmitted is integrated. However, if the channel state of a specificDL CC through which the PDCCH is transmitted is not good, the UE may notreceive downlink data through other DL CCs.

In a type 3, the BS uses an independent PDCCH in each of several DL CCs.A

PDCCH transmitted through one DL CC may carry the scheduling informationof downlink data which is transmitted through the one DL CC, but doesnot carry the scheduling information of downlink data which istransmitted through other DL CCs. The type 3 is very flexible. Further,although the channel state of a specific DL CC is poor, the UE canreceive downlink data through other DL CCs. Accordingly, the type 3 hasa robust system characteristic. However, if the same control informationis repeated every DL CC, unnecessary overhead may occur.

In the type 1 or type 2, when one or more PDCCHs are transmitted for oneLTE-A UE, the PDCCHs can be transmitted through DL CCs different from DLCCs through which PDSCHs corresponding to the PDCCHs are transmitted.Accordingly, it becomes a problem that a PDCCH carries controlinformation which is associated with which one of several DL CCs.

FIG. 18 is a flow diagram showing method of receiving data, performed bya UE, according to an embodiment of the present invention.

Referring to FIG. 18, a UE receives a downlink grant on a PDCCH througha first DL CC from a BS (step S110). The UE receives data based on thedownlink grant through a second DL CC from the BS (step S120).

The UE further receives a second downlink grant on a second PDCCHthrough the first DL CC from the BS. The UE further receives second databased on the second downlink grant through a third DL CC.

FIG. 19 is a flow diagram showing method of transmitting data, performedby a UE, according to another embodiment of the present invention.

Referring to FIG. 19, a UE receives an uplink grant on a PDCCH through afirst DL CC from a BS (step S210). The UE transmits data based on theuplink grant through a first UL CC to the BS (step S220).

Hereinafter, carrier information which indicates a CC associated with aPDCCH is described. The BS can inform the UE of the carrier informationthrough a variety of methods.

(1) Method of Adding a CC Indication Bit Field (CIBF) when Generating anInformation Bit Stream

A CIBF can be added to each DCI format as an information field.

The following table lists examples of CCs indicated by respective CIBFaccording to respective CIBF values.

Table 4

TABLE 4 CIBF CARRIER NUMBER 00 CC #1 01 CC #2 10 CC #3 11 CC #4

Referring to above table, a UE can use four CCs (CC #1, CC #2, CC #3,and CC #4), and the size of a CIBF may be 2 bits. It is, however, to benoted that the above table is only illustrative, and the size of theCIBF and the CCs indicated by the respective CIBFs may be configured invarious ways.

Further, the size of a CIBF may be previously defined between a UE and aBS. Alternatively, a BS may inform a UE of the size of a CIBF throughhigher layer signaling such as a radio resource control (RRC) signaling.Further, the size of a CIBF may be determined according to a UE. Forexample, the size of a CIBF may be determined according to the number ofCCs used by a UE.

The size of a CIBF may be determined according to a DCI format. In thecase of a DCI format for an uplink grant (e.g., a DCI format 0), thesize of a CIBF may be determined according to the number of UL CCs whichare used by a UE. The CIBF of a DCI format for an uplink grant may bereferred to as an uplink (UL) CIBF. The size of a CIBF which is includedin a DCI format for a downlink grant may be determined according to thenumber of DL CCs which are used by a UE. The CIBF of a DCI format for adownlink grant may be referred to as a downlink (DL) CIBF. Accordingly,the size of a UL CIBF may differ from the size of a DL CIBF.Furthermore, the size of a CIBF may vary according to a service type, atransmission mode, etc. of a DCI format. The service type can beclassified into semi-persistent scheduling (SPS) for transmitting voiceover internet protocol (VoIP), etc., dynamic scheduling, and so on. Thetransmission mode may be classified into one antenna transmissionscheme, a transmission diversity scheme, an open-loop spatialmultiplexing scheme, a closed-loop spatial multiplexing scheme, amulti-user (MU)-MIMO scheme, and so on. For example, in the case of aDCI format for the open-loop spatial multiplexing scheme, all CCs arenot used, but only some of the CCs are used. Accordingly, a DCI formatmay not include the CIBF, or the size of the CIBF may be small.

In the case where a UE uses a plurality of CCs, the CIBF can be appliedto all PDCCHs. In the case where a UE uses only one CC, the CIBF may bereserved or may be used for other purposes. Alternatively, when aninformation bit stream is generated, the CIBF may not be included.

(2) Method of Masking Carrier Information to CRC

A BS can indicate carrier information through a specific masking patternfor the CRC of a PDCCH. A UE can determine that the PDCCH is for whichCC based on the CRC masking pattern of the PDCCH. For example, a UEidentifier (ID) can be used in order to mask carrier information to CRC.

A LTE UE is assigned with one UE ID in a cell. A method of assigning aUE ID to a LTE-A UE may be various. A method of assigning a UE ID to aLTE-A UE is described below.

First, a BS may assign a CC-specific UE ID to each UE. That is, a BSassigns an independent UE ID to a UE on a CC basis. A UE is assigned aplurality of UE IDs, and the number of UE IDs is identical to the numberof CC. A BS masks a CC-based UE ID to the CRC of control informationwhich will be transmitted to a UE. For example, a UE can detect a PDCCHbased on a UE ID for a CC #2 in a CC #1. The UE can receive downlinkdata which is transmitted through the CC #2 on the basis of the PDCCH.Although a UE ID is assigned to each LTE-A UE on a CC basis, a BS canefficiently assign a limited number of UE IDs to a number of LTE-A UEsby assigning a different number of CCs to each of the LTE-A UEs.However, this method may increase signaling overhead for UE IDassignment because the UE ID is assigned to each UE on a CC basis.

Second, a BS may assign a UE ID to each UE on the basis of a CC set. Inthe case where a BS assigns m CCs to a LTE-A UE, the BS may classify them number of CCs into n CC sets and may assign a UE ID to each of the nCC sets (where m≧n). The LTE-A UE is assigned the n number of UE IDs.Here, one CC may belong to only one CC set. A BS may flexibly assign aUE ID according to the number of available UE IDs within a cell.Further, when the m number of CCs is classified into the n number of CCsets, “n” can be changed according to time in order to assign the UE IDsmore efficiently. The n number of CC sets may be constructed of the mnumber of CCs in several forms, such as physical layer (or layer 1)signaling or medium access control (MAC) layer (or layer 2) signaling.

Third, a BS may assign a cell-specific UE ID to a LTE-A UE. A UEperforms blind decoding on a PDCCH using always the same UE IDirrespective of CCs assigned thereto. In this case, signaling for UE IDassignment can be simplified. However, the UE cannot determine that thePDCCH is for which CC based on a UE ID which has been masked to a CRC ofthe PDCCH. Accordingly, the BS must inform the UE of carrier informationthrough another method.

(3) Method of Implicitly Indicating Carrier Information

A BS may implicitly inform a LTE-A UE of carrier information. Forexample, a LTE-A UE can determine which PDCCH is for which CC based onthe first control channel element (CCE) index of a CCE aggregation onwhich the PDCCH is transmitted and/or the CCE aggregation level.However, there is a high probability that carrier information may haveerror if the UE does not receive a specific PDCCH.

In accordance with the methods (1) to (3) of a BS informing a UE ofcarrier information, the carrier information may dynamically changeevery subframe.

(4) Method of Transmitting Carrier Information Through RRC Signaling

A BS may semi-statically transmit carrier information to a UE throughRRC signaling. In this case, the carrier information may besemi-statically changed.

As described above, since the BS informs the UE of the carrierinformation, the UE can receive downlink data or transmit uplink data onthe basis of a PDCCH after receiving the PDCCH. The BS can transmitdownlink data to the UE on the PDSCH on the basis of a plurality of CCs.The UE must be able to receive a PDCCH corresponding to the number ofCCs in order to read the downlink data which is transmitted on the PDSCHon the basis of the plurality of CCs.

However, a BS may transmit, to a UE, a PDCCH used for the scheduling ofa PDSCH through a DL CC which is different from a DL CC through whichthe PDSCH is transmitted. Accordingly, a number of PDCCHs for one UE canbe transmitted through one DL CC. A number of the PDCCHs may be fordifferent CCs. A number of the PDCCHs are hereinafter referred to as amulti-PDCCH. The multi-PDCCH is a kind of PDCCH set including aplurality of PDCCHs for one UE and is transmitted through one CC. A BSmay configure one multi-PDCCH or a plurality of multi-PDCCHs for one UE.Accordingly, in the case where a UE uses a plurality of DL CCs, one ormore of the plurality of DL CCs each may use for multi-PUCCHtransmission.

FIG. 20 illustrates a method of configuring a multi-PDCCH.

Referring to FIG. 20, the multi-PDCCH includes two PDCCHs (PDCCH #1 andPDCCH #2). It is, however, to be noted that the above example is onlyillustrative and the multi-PDCCH may include three or more PDCCHs.

A BS generates a first information bit stream according to a DCI formatof the PDCCH #1 and generates a second information bit stream accordingto a DCI format of the PDCCH #2 at step S310. The DCI format of thePDCCH #1 and the DCI format of the PDCCH #2 may be the same or maydiffer from each other. In other words, the DCI format of the PDCCH #1and the DCI format of the PDCCH #2 may be independent from each other.The first information bit stream and the second information bit streammay include respective CIBFs.

The BS attaches a CRC #1 to the first information bit stream and a CRC#2 to the second information bit stream at step S320. A UE ID #1 ismasked to the CRC #1, and a UE ID #2 is masked to the CRC #2. The UE ID#1 and the UE ID #2 may be the same or may differ from each other. Inthe case where the UE ID #1 and the UE ID #2 differ from each other, theUE ID #1 may indicate a CC for the PDCCH #1 and the UE ID #2 mayindicate a CC for the PDCCH #2.

A CRC may be applied to each of PDCCHs constituting a multi-PDCCH.Alternatively, a CRC may be applied to only a multi-PDCCH. For example,a CRC of a multi-PDCCH may be attached to a bit stream in which a firstinformation bit stream and a second information bit stream are combined.For another example, a CRC may be applied to both a multi-PDCCH and eachPDCCH. For example, a CRC of a multi-PDCCH may be further attached to abit stream in which a first information bit stream to which a CRC #1 hasbeen attached and a second information bit stream to which a CRC #2 hasbeen attached are combined. Here, the length of the CRC applied to eachPDCCH and the length of the CRC applied to the multi-PDCCH may differfrom each other.

A joint coding method or a separate coding method can be used as achannel coding method for multi-PDCCH configuration. In the joint codingmethod, bit streams in each of which information bit streamscorresponding to respective PDCCHs are combined are subject to channelcoding together. A UE can obtain plural pieces of control informationthrough single channel decoding. In the separate coding method,information bit streams corresponding to respective PDCCHs areindividually subject to channel coding, thereby generating respectivecoded bit streams. A multi-PDCCH can be configured by packing aplurality of coded bit streams. Here, PDCCHs constituting themulti-PDCCH preferably have the same channel coding rate.

CCE aggregations on which respective PDCCHs constituting a multi-PDCCHare transmitted may be consecutive to each other or may be separatedfrom each other.

FIG. 21 shows an example in which a multi-PDCCH is transmitted in amulti-carrier system.

Referring to FIG. 21, the multi-PDCCH for a UE #1 is transmitted througha CC #1. PDCCHs constituting the multi-PDCCH are for different CCs.

FIG. 22 shows another example in which a multi-PDCCH is transmitted in amulti-carrier system.

Referring to FIG. 22, each of PDCCHs constituting the multi-PDCCHincludes a CIBF indicative of carrier information. The multi-PDCCH for aUE #1 is transmitted through a CC #1. Here, the multi-PDCCH includes aPDCCH for the CC #1 and a PDCCH for a CC #2. A PDCCH for the UE #1 istransmitted through a CC #L.

A CC through which a multi-PDCCH is transmitted is described below.

A multi-PDCCH may be transmitted through only a specific CC (refer toFIG. 21). This method corresponds to the method of transmitting amulti-PDCCH according to the type 2 of FIG. 17. Alternatively, amulti-PDCCH may be transmitted through any one of a plurality of CCs.This method corresponds to the method of transmitting a multi-PDCCHaccording to the type 1 of FIG. 17. Here, in order to maximize afrequency diversity gain, a CC through which a multi-PDCCH istransmitted may be changed into a specific pattern according to thehopping rule which is previously agreed between a BS and a UE.

In an alternative method, the transmission method according to the type1 and the transmission method according to the type 2 may be adaptivelyused according to channel condition. For example, a UE which is in ahigh-speed mobile environment is difficult to determine which CC has agood channel condition. In this case, the UE may transmit a multi-PDCCHaccording to the type 1. Here, a CC through which the multi-PDCCH istransmitted can be determined in accordance with the hopping rule.Accordingly, a frequency diversity gain can be obtained. On the otherhand, a UE which is in a low-speed mobile environment can determinewhich CC has a good channel condition through a variety of feedbackchannels. The UE may select a specific CC according to time and transmita multi-PDCCH through the selected CC. Here, a BS must inform the UE ofinformation about the CC through which the multi-PDCCH is transmitted.

A CC set through which a multi-PDCCH is transmitted may be configured.For example, in the case where a UE uses an L number of CCs, themulti-PDCCH can be transmitted through only a CC set which is defined tobe the N number of CCs (N<L). Accordingly, the complexity of blinddecoding of a UE can be reduced. Here, a number of PDCCHs may bedistributed and transmitted through respective CCs within the CC set.Information about a CC set may be previously defined, or a UE may beinformed of information about a CC set through RRC signaling.

FIG. 23 shows still another example in which a multi-PDCCH istransmitted in a multi-carrier system.

Referring to FIG. 23, a multi-PDCCH for a UE #1 is transmitted through aCC #1. PDCCHs are not transmitted through a CC #2. That is, only a PDSCHcan be transmitted through the CC #2.

As described above, in the case where a multi-PDCCH is transmitted, CCsthrough which PDCCHs are not transmitted, but through which only PDSCHscan be transmitted can be configured. A CC through which only a PDSCHcan be transmitted can be used along with other CC through which a PDCCHassociated with the PDSCH is transmitted. Further, in the case where aCC through which only a PDSCH can be transmitted is configured, a UE maybe configured not to transmit or receive any information during aspecific subframe transmitted through the CC.

FIG. 24 shows an example in which PDCCHs are transmitted in amulti-carrier system.

Referring to FIG. 24, a UE uses an L number of DL CCs (DL CC #1, DL CC#2, . . . , DL CC #L) and a U number of UL CCs (UL CC #1, UL CC #2, . .. , UL CC #U). When the L number of DL CCs is the same as the U numberof UL CCs, the DL CCs and the UL CCs have a symmetric structure. Whenthe L number of DL CCs is different from the U number of UL CCs, the DLCCs and the UL CCs have an asymmetric structure.

A BS transmits three PDCCHs to a UE through the DL CC #1. One of thethree PDCCHs is for the DL CC #1, another of the three PDCCHs is for theDL CC #L, and yet another of the three PDCCHs is for the UL CC #2. Eachof the PDCCHs may include a CIBF indicating carrier information. In thecase where the L number of DL CCs differs from the U number of UL CCs,the size of a UL CIBF may differ from the size of a DL CIBF. Forexample, in the case where the U number of UL CCs is smaller than the Lnumber of DL CCs, the size of a UL CIBF may be smaller than the size ofa DL CIBF.

FIG. 25 shows an example in which CC subsets are set.

Referring to FIG. 25, a UE may be assigned with an L number of DL CCs(DL CC #1, DL CC #2, . . . , DL CC #L). A CC super set includes the Lnumber of DL CCs. A CC subset including the DL CC #2 and the DL CC #Lmay be set from the CC super set. It is, however, to be noted that theabove example is only illustrative and a CC subset may be set from a CCsuper set in various ways. A UE may use only DL CCs included in a CCsubset.

The number of DL CCs required for the UE to which the L number of DL CCshas been assigned may vary according to channel condition or a servicetype. However, if the L number of DL CCs is always set for a UE, the UEmust monitor all the DL CCs and measure channels for all the DL CCs. Itmakes the UE unnecessarily consume its power. Accordingly, if a CCsubset is set, a UE has only to monitor only DL CCs belonging to the CCsubset and to measure channels for the DL CCs. In this case, unnecessarycalculation complexity and power consumption in the UE can be reduced.

A BS may inform the UE of information about the CC subset through RRCsignaling, a PDCCH, a broadcast message, or the like. Information aboutthe CC subset may indicate CCs which constitute the CC subset using abitmap. If a bitmap is used, flexibility in setting a CC subset can beincreased.

It is hereinafter assumed that a UE uses a CC subset and a PDCCHincludes a CIBF.

In order to reduce the size of the CIBF, the CIBF may indicate CCs onthe basis of the CC subset not a CC super set. An example in which, asin FIG. 25, the CC subset includes DL CC #2 and DL CC #L is describedbelow. For example, when the CIBF of a PDCCH is 2, the PDCCH is for theDL CC #L. In the case where the UE does not use the CC subset, the PDCCHhaving the CIBF of 2 is for the DL CC #2. However, in order to reducecomplexity, a CIBF may always have the same size irrespective of whethera CC subset is used or not.

Although DL CCs have so far been described, UL CCs may also be limitedto a CC subset.

The length of a CP in each CC is described below.

In a multi-carrier system, CCs have the same subframe length, but havedifferent center frequencies. In particular, in the case where adjacentCCs are physically discontinuous in the frequency domain, channelcharacteristics between the CCs may differ. Furthermore, each of the CCsmay have different delay spread. Accordingly, CPs having differentlengths may be used every CC or every CC set. The number of OFDM symbolswithin one subframe changes according to the length of a CP. A case inwhich a first CC with an SCH and a second CC without an SCH are assignedto one UE is hereinafter assumed. The UE can find the length of a CP ofthe first CC through the SCH. The UE can obtain the length of a CP ofthe second CC through RRC signaling which is transmitted through thefirst CC or a control channel, such as a PDCCH transmitted through thefirst CC.

FIG. 26 is a block diagram showing wireless communication system toimplement an embodiment of the present invention. A BS 50 may include aprocessor 51, a memory 52 and a radio frequency (RF) unit 53. Theprocessor 51 may be configured to implement proposed functions,procedures and/or methods described in this description. Layers of theradio interface protocol may be implemented in the processor 51. Thememory 52 is operatively coupled with the processor 51 and stores avariety of information to operate the processor 51. The RF unit 53 isoperatively coupled with the processor 11, and transmits and/or receivesa radio signal. A UE 60 may include a processor 61, a memory 62 and a RFunit 63. The processor 61 may be configured to implement proposedfunctions, procedures and/or methods described in this description. Thememory 62 is operatively coupled with the processor 61 and stores avariety of information to operate the processor 61. The RF unit 63 isoperatively coupled with the processor 61, and transmits and/or receivesa radio signal.

The processors 51, 61 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit, data processing deviceand/or converter which converts a baseband signal into a radio signaland vice versa. The memories 52, 62 may include read-only memory (ROM),random access memory (RAM), flash memory, memory card, storage mediumand/or other storage device. The RF units 53, 63 include one or moreantennas which transmit and/or receive a radio signal. When theembodiments are implemented in software, the techniques described hereincan be implemented with modules (e.g., procedures, functions, and so on)that perform the functions described herein. The modules can be storedin memories 52, 62 and executed by processors 51, 61. The memories 52,62 can be implemented within the processors 51, 61 or external to theprocessors 51, 61 in which case those can be communicatively coupled tothe processors 51, 61 via various means as is known in the art.

As described above, in a multi-carrier system, a BS can efficientlytransmit a PDCCH. A UE can receive downlink data or transmit uplink dataefficiently on the basis of a PDCCH. A method and an apparatus ofeffectively receiving data are provided. Further, backward compatibilitywith a single carrier system can be maintained. Accordingly, an overallsystem performance can be improved.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

1-9. (canceled)
 10. A method of communication in a wirelesscommunication system, carried in a user equipment (UE), the methodcomprising: monitoring a set of physical downlink control channel(PDCCH) candidates in a subframe; when a PDCCH candidate is successfullydecoded, acquiring a grant on the successfully decoded PDCCH candidate,the grant comprising a resource assignment and a component carrier (CC)indication field, the CC indication field indicating a CC scheduled forthe resource assignment; and receiving or transmitting data using theresource assignment through the CC indicated by the CC indication field.11. The method of claim 10, wherein the PDCCH candidate is successfullydecoded if no CRC error is detected after de-masking the CRC of thePDCCH candidate with the UE's identifier.
 12. The method of claim 11,further comprising: receiving, from a base station, a message indicatingat least one DL CC to monitor the set of PDCCH candidates.
 13. Themethod of claim 12, wherein the at least one DL CC is different from theDL CC indicated by the CC indication field.
 14. The method of claim 10,further comprising: receiving, from a base station, a message indicatingwhether the CC indication field is present in the grant.
 15. The methodof claim 14, wherein the message is a radio resource control (RRC)message.
 16. The method of claim 10, wherein a size of the CC indicationfield is 3 bits.
 17. The method of claim 10, wherein the subframeincludes a plurality of OFDM symbols in time domain and is divided intoa control region and a data region, and the set of PDCCH candidates ismonitored in the control region.
 18. A UE comprising: a radio frequency(RF) unit for transmitting and receiving a radio signal; and a processoroperatively coupled with the RF unit and configured for: monitoring aset of physical downlink control channel (PDCCH) candidates in asubframe; when a PDCCH candidate is successfully decoded, acquiring agrant on the successfully decoded PDCCH candidate, the grant comprisinga resource assignment and a CC indication field, the CC indication fieldindicating a CC scheduled for the resource assignment; and receiving ortransmitting data using the resource assignment through the CC indicatedby the CC indication field.
 19. The UE of claim 18, wherein the PDCCHcandidate is successfully decoded if no CRC error is detected afterde-masking the CRC of the PDCCH candidate with the UE's identifier. 20.The UE of claim 19, wherein the processor is further configured for:receiving, from a base station, a message indicating at least one DL CCto monitor the set of PDCCH candidates.
 21. The UE of claim 20, whereinthe at least one DL CC is different from the DL CC indicated by the CCindication field.
 22. The UE of claim 18, wherein the processor isfurther configured for: receiving, from a base station, a messageindicating whether the CC indication field is present in the grant. 23.The UE of claim 22, wherein the message is a radio resource control(RRC) message.
 24. The UE of claim 18, wherein a size of the CCindication field is 3 bits.
 25. The UE of claim 18, wherein the subframeincludes a plurality of OFDM symbols in time domain and is divided intoa control region and a data region, and the set of PDCCH candidates ismonitored in the control region.